Born from the conflict between Selûne and Shar, the original Mystryl was a Chaotic Neutral entity that was relatively flighty and carefree; A real rainbow chick.

She either created The Weave, embodied it, or was a side effect of its creation, but in any case she was in charge of it. Her attitude towards the use of the weave was "do as you please, but just don't piss me off", which meant that magic wielders could perform spectacular feats that were unheard of in later times.

However, this also opened the weave up to abuse, where one particular spellcaster called Karsus decided to create a 12th level spell that would give himself control over the weave itself.

The spell worked... for all of five minutes. While he wrestled control over the weave from the goddess; he couldn't actually control it and caused magic all over the world to get screwed up. Destroying his own empire of floating cities and essentially reverting everything back to the stone ages.

Mystryl figured she'd rather die than let some idiot ruin the fundamental forces of magic, so she sacrificed herself to restabilise the weave and gave birth to her successor Mystra

The next Goddess was an entirely different creature. Immediately imposing rules and restrictions on the weave, preventing those kinds of abuses from ever happening again. Capping spellcasting ability to a maximum of 9th level spells, while changing how magic works.

Furthermore, upon realising that you can do pretty much anything using the power of magic and that Mystra is the god of "all magic" in an uncharacteristically active move, the overdeity Ao manipulated events to ensure that Mystra was forced to spread her power around so that she did not become "all powerful" and either upset the balance or try something stupid like usurp Ao himself. So the second Mystra was made to be more more active and had to spread her power around.

She fell in love with a human Wizard called Azuth and sponsored him as the first Magister giving him a humongous power boost and helping him on his journeys through the realms, to the point where he became a god himself, then continuing the tradition of magisters by starting a chain of them.

Then she started appointing Chosen amongst mortal followers, granting them a portion of her divine strength. Though the gifts not as powerful as the Magister outright, there were a lot more of them. But unfortunately she realised that sometimes, all this power just burned through mortals if they weren't already strong enough to wield it, or some of them became corrupted by the power and became tyrants. To counter this "power corrupts" problem, she decided to have children and spawned a set of daughters who were imbued with the abilities of her Chosen from birth and raised to appreciate their gifts.

All told, Mystra had about half of her divine power wrapped up between her Chosen, her Magister and her servant Azuth. Yet even with much of her power spread across the cosmos, she was still the most powerful deity in the Pantheon.

So when the Tablets of Fate were stolen from Ao and he kicked all the Gods except Helm out into the mortal realm until somebody owned up to it; Mystra figured she could just barge her way back in.

Unfortunately; powerful mortal avatar of a god that she was, in this instance she was still no god. So Helm got the upper hand and cut Mystra down in front of an city's worth of audience.

The third incarnation began life as a mortal woman called Midnight who was also a travelling companion of mortal versions of Kelemvor and Cyric as well as cleric of Sune called Adon.

Midnight was pivotal in the recovery of the Tablets and was rewarded with Mystra's portfolio, creating a new goddess of magic but taking the old name.

This third Mystra, under the tutelage of Azuth, took a while to learn the ropes of her new role. She tried to maintain her original human outlook while doing the job and started shifting the church of Mystra from Neutrality over to Good while attempting to influence how magic was being used in the realms, unfortunately while this sounds noble it wasn't actually the best of ideas. She, along with her former mortal-turned-god companions Kelemvor and Cyric were placed on trial by the other gods as being negligent in their duties as deities by incompetence through humanity. Cyric managed to argue that his lolrandumb behaviour was fitting for the chaotic God of Lies and Betrayal (even if it was pissing off everyone else in the Pantheon by risking drawing Ao's attention again); Mystra and Kelemvor realized they were bungling their new jobs and agreed to sort their shit out. In the end, all three started playing by the divine rulebook, i.e: "If it's not in your portfolio then you can't do it, final fucking period!"

She was killed when Shar and Cyric teamed up. The whole Spellplague was caused because there was no god of magic keeping the Weave in check. Only the minor god of note keeping: Deneir managed to keep the universe from turning itself inside out by writing himself into the Weave and causing all the mechanical changes to magic rules in 4e.

The return of Mystra in 5e was actually done quite elegantly: playing on plot hooks laid several editions earlier. Even though Mystra as a deity was "dead" she lived on as a Vestige. Despite this, remember that half her power was invested elsewhere in the cosmos.

Her most powerful Chosen, a woman called The Simbul, gathered up a whole bunch of magical energy and combined it with that of Elminster who in turn used it to resurrect the goddess, creating a new amalgam of 1e Mystra & 2/3e Midnight all in one package.

"Once in every generation a slayer Spellfire Wielder is born..." or at least they thought there was. Turns out there are a lot more Spellfire Wielders than anyone knew about.

Probably because the people who have it; generally don't want it, while those that don't have it; usually want it. Unfortunately, you can only ever obtain Spellfire at birth, but that hasn't stopped unscrupulous arcanists from attempting to gain it anyway.

Spellfire is raw magic than can be shaped and applied in various ways, but not quite as useful or all powerful as "spells".

The bearer of Spellfire acts like a living Rod of Absorption, being able to nullify incoming magical spells and store the energy inside them, then they can either fling it back out as raw destructive power, or to heal others with a touch.

So at its most basic, Spellfire is a powerful tool for anyone, regardless of your class, since it doesn't require you to actually learn how to use it. It also explains how so few people actually know they have it, since the common person in the Forgotten Realms doesn't usually come into contact with magic.

Spellfire can be mastered though: With the proper training a person can become a Spellfire Channeler, who can increase their capacity to store raw magic inside them, building up a visible glow as they accumulate even more power, but becoming risky to be around as the Spellfire has a risk of spontaneous overload. However, they can use their power in different ways, such as the ability to blast more often, heal more effectively, drain magical items of power, knock incoming projectiles out the air, melt weapons that strike them, FLY, and even go nuclear with their discharge rather than fling it precisely.

It should be noted that the Spellthief class is very similar to Spellfire, and aught to be a viable alternative to those who want to be like Spellfire Wielders, but weren't born that way. Comparatively, Spellthieves can recast the spell that was used on them, while Spellfire is converted into raw energy which can only be used in simple ways, and a Spellthief can take levels in the class at any point in thier career rather than excluded from it by birth. However, Spellthieves still suck though. Spellfire Wielding is a feat that can be taken regardless of class (but still only at character creation), Spellfire also has a 100% success rate of absorbing magic, rather than a mere chance like the thief. Stored Spellfire has no use-by duration, unlike stored Spellthief magic which must be used quite quickly or it is lost, and Spellfire use is a supernatural ability, rather than a spell or spell-like ability so it cannot be interrupted or resisted. There is no class that couldn't be made better with Spellfire, except the Spellthief because it make their abilities redundant.

As part of Mystra's need to spread her power around, she spawned quite a few of Chosen, rather than having one like any normal god.

Their ranks include Elminster, Khelben Blackstaff Arunsun (Voiced by Patrick Stewart) as well as a set of seven sisters directly descended from Mystra.

While they are all powerful in their own right, there is no strict need for them to be Arcanists, though most of them have some ability. Nor are they given as much power or responsibility as the Magister (see below), even though the most powerful chosen: "The Simbul" is easily the most powerful spellcaster in the realms in her own right, regardless of any external "granted" power. So the purpose of the chosen is to simply to hang on to Mystra's power for safekeeping.

Each one of them us immune to aging, disease, poison, disintegration and sleep. They gain an immunity to one spell of each level (like the Magister) as well as gaining one spell of each level on their spells list, good if they are Sorcerers, less so for Wizards who can just learn spells anyway.

They also gain the use of Silver Fire which is a lesser form of Spellfire described above. It has a range of minor uses: like generating light and warmth, but can only be used in the more powerful ways similar to true spellfire just once an hour or so as it had to recharge. It cannot drain incoming magic, nor can it reach the supreme heights of charged Spellfire, but at least they don't have to rely on draining magic to use it.

Mystra's foremost servant in the realms is the Magister and comes with the responsibility for promoting the understanding and availability of magic to the people. Unlike the Chosen of Mystra who need not necessarily be spellcasters at all, the Magister is always an arcane spellcaster, and typically always a Wizard as well; because other arcanists like Sorcerers or unusual classes like Warmages or Dread Necromancers are too limited to be able to fulfil the job requirements.

It used to be that the position of Magister was transferrable by defeating the holder in a mage-duel, which would be an impressive feat considering the sheer number of buffs the Magister gets by default. But then it ended up that the Magister would just be hounded by a queue of ambitious dickhead wizards looking for more power with no guarantee they would actually be any good in the job, thus the Magister would become a recluse and hide rather than doing the job themselves. Azuth put a stop to transfer by mage-duels and instead sponsors candidates he thinks is worthy. Though that doesn't stop the dickheads who never got the memo from lining up, hoping to have a go anyway.

When appointed, the personal sigil of the Magister appears in every temple of Mystra of ten minutes and every magic user in the world feels a ripple, informing them the position has changed. Mystra has also decreed that her priests must reveal the identity of the Magister if they know it, subject to a suitable donation to the church of course.

Thankfully, the new Magister gets a 10 day grace period, where they get teleported to a pocket dimension so they can get instructed by heralds of Azuth in the requirements of the role, while usually getting access to the spell book of every previous holder of the role, a significant prize in itself.

When they get released back into the general population, because it is their duty to spread magic around, they are able to recover half of the experience cost for items they create if they give them away, turning them into the arcane Santa Claus.

Other abilities include:

A complete immunity to enchantment, and a +2 holy resistance to all magical effects

Spell Resistance 20

The ability to sense wild or dead magic zones

They can hear whenever their name gets mentioned anywhere in the world along with the following nine words and the general direction of the speaker

constant Mindblank protection

The ability to cast levitate, true seeing, read magic and feather fall at will, and cast dimension door and water walk 6/day

They can pass through ANY magical barrier of 6th level or lower without ill effect

Complete immunity to a chosen spell of each spell level

The ability to imbue permanency on any spell they know

They get Spell Focus for every spell

They prepare their spells in ten minutes.

So no wonder everyone wants the job...

Holders of the role can step down from the role voluntarily and allow someone to take it from them (presumably subject to Mystra's and Azuth's approval). But considering it usually takes some substantial magical power to get the role in the first place, the gods would like you to keep serving. In which case the previous holder can relegate themselves to being a Chosen of Azuth, or become a sentient magical item, or start-over by being reincarnated as a whole new person with magical gifts.

Mystra has a few Militant Orders of non-Arcanists dedicated to her service: The Order of the Shooting Star(Rangers) and The Knights of Mystic Fire(Paladins).

The Shooting Star Rangers lose the ability to gain an animal companion, the bonus Endurance feat and the Trackless Step ability, but in trade they get one bonus spell slot of each spell level, as well as a +2 caster level bonus when using ranger spells and also get a slightly expanded spell list.

Mystic Fire Paladins lose the ability to Turn Undead and cannot remove disease. But they augment their Smite ability to stagger victims and make it difficult to cast spells unless they pass a concentration check. They also get bonus spell slots for each spell level and the ability to dispel magic as part of a melee attack once per day.

Both orders can take a feat that allows them to take Wizard/Sorcerer spells in their Ranger/Paladin spell list without any other restriction that having to use a spell book to keep them in. Which can radically improve how useful these classes function in a normal game.

Mystra's specialist priests are the Dweomerkeepers. As expected for a deity of magic they are familiar with both arcane and divine spellcasting, but are not equally good at both in the same way that a Mystic Theurge would be.

Entry requirements include being able to cast level 2 Arcane and Divine spells and access to the Magic (or Spell) domain, so Druids need not apply. They get a casting increase every level, but have to choose which class to use. Meaning unless you have the "Precocious Apprentice" feat from Complete Arcane you are essentially giving up three levels of your primary spellcasting class to use this prestige class.

The benefits for doing so allow them to choose a spell at every other level and be able to spontaneously convert prepared spells -whether Arcane or Divine- into those chosen spells. Which can be very useful if you choose combat spells, meaning you can prepare your slots for utility encounters without being defenceless.

Other class features are interchangeable: in Faiths and Pantheons (FR 3.0) they gain bonus feats similar to a wizard, but in Complete Divine (Core 3.5) they gain the ability to cast some prepared spells as supernatural abilities, rendering them impossible to counter or interrupt. Your GM should decide which version of the class is most appropriate to your campaign, but the FR version is more difficult to qualify for, needing more feats and for your race to be human

Mystra's domain of Dweomerheart is/was a magical mountain located in Eronia the second layer of Elysium. Dweomerheart is located at the summit where all spells cast are automatically Enlarged, Extended and Empowered without any modification to the spell level or casting time. Her servants Azuth, Savras and Velsharoon all occupy various caverns and chambers within the mountain.

Like most petitioners of Elysium, those who end up here in the afterlife get to keep some memories of their former life. In fact, petitioners of the gods of magic also gain spell resistance and damage reduction, making this realm a veritable fortress of spellcasters.

Unfortunately with the 4th Edition Spellplague, the realm exploded upon the death of Mystra's, taking the other resident gods with it.